Generic NMEA GPS Receiver

1. Synopsis

2. Description

This driver supports GPS receivers with the $GPRMC, $GPGLL,
$GPGGA, $GPZDA and $GPZDG NMEA sentences by default. Note that
Accord’s custom NMEA sentence $GPZDG reports using the GPS timescale,
while the rest of the sentences report UTC. The difference between the
two is a whole number of seconds which increases with each leap second
insertion in UTC. To avoid problems mixing UTC and GPS timescales, the
driver disables processing of UTC sentences once $GPZDG is received.

The driver expects the receiver to be set up to transmit at least one
supported sentence every second.

The accuracy depends on the receiver used. Inexpensive GPS models are
available with a claimed PPS signal accuracy of 1 µs or better relative
to the broadcast signal. However, in most cases the actual accuracy is
limited by the precision of the timecode and the latencies of the serial
interface and operating system.

If the operating system supports PPSAPI
(RFC 2783), the flag1 1 option
enables its use.

If there is a /dev/gpsppsu it is tried first for PPS, before /dev/gpsu.

The various GPS sentences that this driver recognises look like this
(others are quietly ignored):

Important caveats: If your NMEA device does not ship GPZDA, you cannot
use it to run autonomously without a check peer, or expect recovery
from a trashed system clock. Also, dates from old NMEA devices are
vulnerable to era wraparound; the NMEA driver has an internal trick
that attempts to detect this, but one or more "g" suffixes on your
time1 option may be useful as a workaround.

Table 2. NMEA data items

Symbol

Meaning and Format

UTC

Time of day on UTC timescale. Hours, minutes and seconds [fraction
(opt.)]. (hhmmss[.fff])

POS_STAT

Position status. (A = Data valid, V = Data invalid)

LAT

Latitude (llll.ll)

LAT_REF

Latitude direction. (N = North, S = South)

LON

Longitude (yyyyy.yy)

LON_REF

Longitude direction (E = East, W = West)

SPD

Speed over ground. (knots) (x.x)

HDG

Heading/track made good (degrees True) (x.x)

DATE

Date (ddmmyy)

MAG_VAR

Magnetic variation (degrees) (x.x)

MAG_REF

Magnetic variation (E = East, W = West)

FIX_MODE

Position Fix Mode (0 = Invalid, >0 = Valid)

SAT_USED

Number of Satellites used in solution

HDOP

Horizontal Dilution of Precision

ALT

Antenna Altitude

ALT_UNIT

Altitude Units (Metres/Feet)

GEO

Geoid/Elipsoid separation

G_UNIT

Geoid units (M/F)

D_AGE

Age of last DGPS Fix

D_REF

Reference ID of DGPS station

GPSTIME

Time of day on GPS timescale. Hours, minutes and seconds [fraction
(opt.)]. (hhmmss[.f])

3. The 'mode' byte

Specific GPS sentences and bitrates may be selected by setting bits of the 'mode' in the refclock configuration line:

Table 3. mode byte bits and bit groups

Bit

Decimal

Hex

Meaning

0

1

1

process $GPRMC

1

2

2

process $GPGGA

2

4

4

process $GPGLL

3

8

8

process $GPZDA or $GPZDG

7

128

0x80

Write the sub-second fraction of the receive time stamp to the clockstat
file for all recognized NMEA sentences. This can provide a useful
value for fudge time2
Caveat: This will fill your clockstat file rather fast. Use it only
temporarily to get the numbers for the NMEA sentence of your choice.

8

256

0x100

process $PGRMF

9-15

0xFE00

reserved - leave 0

16

65536

0x10000

Append extra statistics to the clockstats line. Details below.

The default (mode 0) is to process all supported sentences at a
linespeed of 9600 bps, which results in the first one received and
recognized in each cycle being used. If only specific sentences should
be recognized, then the mode byte must be chosen to enable only the
selected ones. Multiple sentences may be selected by adding their mode
bit values, but of those enabled, only the first received sentence
in a cycle will be used. Using more than one sentence per cycle is
impossible, because

there is only the time2 option available to compensate
for transmission delays but every sentence would need a different one

and

using more than one sentence per cycle overstuffs the internal data
filters.

The driver uses 9600 bps by default, unless
NTPsec was built in Classic strict-compatibility mode, in which
case the old default of 4800 bps is used. Faster bitrates can be
selected using the baud option.

Caveat: Using higher line speeds does not necessarily increase the
precision of the timing device. 9600 bps is useful to accommodate an
amount of data that does not fit into a 1-second cycle at 4800 bps;
some GPSes (especially when emitting skyview sentences) emit more than
this. Any device that is exclusively used for time synchronisation
purposes should be configured to transmit the relevant data only,
e.g. one $GPRMC or $GPZDA per second; this will slightly reduce
jitter by reducing variation in transmission times.

For backwards compatibility with NTP Classic, some bits of the mode
byte can be used to set the device baud rate. This is deliberately
left undocumented here, as new configurations should use the "baud"
keyword. In future releases the compatibiility shim may be removed.

4. Monitor Data

The last GPS sentence that is accepted or rejected is written to the
clockstats file and available with ntpq -c clockvar. (Logging the
rejected sentences lets you see/debug why they were rejected.) Filtered
sentences are not logged.

If the 0x10000 mode bit is on and clockstats is enabled, several extra
counters will be appended to the NMEA sentence that gets logged. For
example:

Number of sentences rejected
because they were marked invalid (poor signal)

8

0

Number of sentences rejected because of bad
checksum or invalid date/time

9

64

Number of sentences filtered by mode bits or same second

10

0

Number of PPS pulses used, overrides NMEA sentences

The clock identification in field 3 is normally the driver type and
unit, but if your ntpd was built in strict Classic compatibility
mode it will be a magic clock address expressing the same information
in a more opaque way.

Sentences like $GPGSV that don’t contain the time will get counted in
the total but otherwise ignored.

Configuring
NMEA Refclocks might give further useful hints for specific hardware
devices that exhibit strange or curious behaviour. Note that it
describes NTP Classic and the old refclock configuration syntax, and
assumes the old default speed of 4800 bps.

To make a specific setting, select the corresponding decimal values from
the mode byte table, add them all together and enter the resulting
decimal value into the clock configuration line.

5. Setting up the Garmin GPS-25XL

Switch off all output by sending it the following string.

"$PGRMO,,2<cr><lf>"

Now switch only $GPRMC on by sending it the following string.

"$PGRMO,GPRMC,1<cr><lf>"

On some systems the PPS signal isn’t switched on by default. It can be
switched on by sending the following string.

"$PGRMC,,,,,,,,,,,,2<cr><lf>"

It is likely any surving instances of this hardware will have
era-rollover issues when reporting dates. One or more "g" suffixes
on your time1 option may be useful as a workaround.

6. Driver Options

unitnumber

The driver unit number, defaulting to 0. Used as a distinguishing
suffix in the driver device name.

time1time

Specifies the PPS time offset calibration factor, in seconds and
fraction, with default 0.0.

time2time

Specifies the serial end of line time offset calibration factor, in
seconds and fraction, with default 0.0.

stratumnumber

Specifies the driver stratum, in decimal from 0 to 15, with default 0.

refidstring

Specifies the driver reference identifier, an ASCII string from one to
four characters, with default GPS.

7. Configuration Example

8. Known bugs

If your GPS has firmware made more than 1024 weeks (19 years and 36
weeks) in the past, its internal date counter will almost certainly
wrap around and generate spurious timestamps. Beginning in January
2018, newer GPSes may have a longer wraparound (8192 weeks, or 157
years and 28 weeks) but it is not safe to bet that any given receiver
will have firmware updated to take advantage of this.

This problem is fundamental and cannot be compensated for in code
without relying on the accuracy of the local system clock, which
is exactly what an NTP implementation may not do without risking
perverse failure modes (especially at startup time). The only
remedy is not to use ancient GPS hardware.